Concerning many medical products, absorbability, metabolizability, solubility, etc. of a drug are associated with hydrophobicity of a compound used as the drug. There is a conventional technique of determining logD (water-octanol partition coefficient) as an index to indicate the hydrophobicity of a compound (e.g., cf. Patent Documents 1-3 below).    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-124756    Patent Document 2: Japanese Patent No. 3444872    Patent Document 3: Japanese Patent No. 3523207
A method of determining logD is to calculate logD on the basis of a retention time in HPLC (High Performance Liquid Chromatography), as described in Patent Documents 1-3 above. However, this method obtains the calculated logD as an estimate value, and thus has a problem that it is more likely to produce an error than a method using actually measured values as described below.
Another method of determining logD is a shake-flask method. The shake-flask method is to put a compound as a measured object, water, and octanol in a flask, to shake the mixture, to measure a concentration of the compound in water and a concentration of the compound in octanol, and to calculate logD on the basis of the measured values.
The shake-flask method involves the step of measuring the concentrations in the respective solutions, which may be a method of separating water and octanol and measuring the concentrations in the separate solutions. However, this separating operation is cumbersome and does not suit lump-sum handling of multiple analytes. The separating operation originally involves adsorption on a container or the like and could cause error.
There is another method of sampling each of an aqueous solution and octanol by means of a liquid handler or the like. However, since the water layer is located below the octanol layer, this method normally has a problem that octanol is mixed in the aqueous solution during sampling thereof to cause contamination. Particularly, when logD is a large value in the range of about 4-6, the concentration in the aqueous solution is 1/104 to 1/106 of the concentration in octanol, and mixing of a small amount of octanol will pose a serious problem in the calculation of logD.